The present invention relates to surge protection devices and more particularly to a surge protector and test fixture with triple current sensing and voltage pickoff.
Many military radio communications and other electronic systems have a requirement to survive the electromagnetic pulse (EMP) that would result from a nuclear detonation at high altitude. Nuclear detonations at high altitude are known to cause intense bursts of electromagnetic energy that will illuminate very large areas of the Earth's surface. Electronic equipment within the area illuminated by the EMP is susceptible to damage from the incident EMP wavefront. In general, the susceptible equipment is contained within electro-magnetically shielded enclosures such as equipment shelters. Although such equipment may be electromagnetically shielded, energy sufficient to cause damage may be coupled into the enclosure. The primary mechanism by which energy from an EMP event may enter the equipment is conductive penetration to the wall of the enclosure by exterior cables. Typically these cables connect equipment at varying distances from the enclosure. These distant equipments may include AC power generating equipment, antenna masts, or other shelterized equipment. The cables connecting the physically separated items may behave as antennas which respond to a incident EMP wavefront causing voltage transients to appear at the equipment interfaces. This equipment, unless protected, may be damaged by the EMP event.
Methods available to protect shielded equipment from EMP induced damage are generally referred to as treatment of penetrations. These methods include shielding of cables and the use of Terminal Protection Devices (TPD's). Effective shielding of cables requires that all conductors comprising the cable (i.e., individual wires and wire pairs) be completely surrounded by a metal shield which is continuous with and circumferentially bonded to the walls of the equipment enclosures at both ends of the cable. When end to end shielding of exterior cabling is not possible, TPD's should be used. TPD's are of two general categories; linear and non-linear. The term linear TPD refers to a broad range of commercially available encapsulated filters enclosed in metal shields; these are grounded to the walls of the shielded enclosure in which the protected equipment is located. Examples include AC power line RFI filters, data line filters, and coaxial RF high pass filters. The term non-linear TPD refers to a broad range of devices which limit voltage by clamping or by arc breakdown. Examples of the former are metal oxide varistors (MOV's) and bipolar zener diodes. The latter category are gas discharge tubes or spark gaps. These protect equipment by creating a momentary short circuit across the input to the equipment when the amplitude of an offending transient exceeds the voltage breakdown threshold of the device. At breakdown, an arc discharge occurs within the device which provides a path to ground for currents induced on an exterior conductor by the EMP event. The arc discharge persists for the duration of the transient that caused the voltage breakdown
Many gas discharge devices are contained within a coaxial package and are advertised as fast acting devices. These were devised to provide protection to RF transmitters and receivers in the HF and VHF frequency ranges. For these systems, the spectrum of an incident EMP wavefront contains frequency components within the passband of HF and VHF antennas. When the antenna is excited from an incident wavefront, a high voltage fast rise-time transient will appear at the equipment. For the equipment to be protected, the protective device must have sufficient activation speed to limit the amplitude and duration of the transient below previously established damage threshold. If the protective device has sufficient speed in relation to the voltage rate of rise (volts/second) of the transient, it will enter breakdown and the equipment will be protected; if not, the device may fail to activate and the equipment may be damaged. It is well known that the activation or breakdown voltage of gas tube TPD's increases as the rate of rise (volts/time) of an applied voltage stress increases. What is not well known is that the breakdown voltage vs rate of rise characteristic may change for some devices with time and usage and the tendency to vary is manufacturer dependent. In other words, a device, when installed in a system, may fail to activate at a crucial time. Thus, there is a great need to test a large number of devices from different manufacturers to determine the activation characteristics both initial and long term. Prior to installation in a system, protective devices should be pulse tested so that marginal devices can be rejected. After installation, a device should be periodically pulse tested to determine if replacement is necessary.